First, I want to say that I was impressed with the visual upgrade in AoE: DE.
That said, I think there is a good consensus that the devs should have, at the very least, modernized some aspects of the gameplay, like seeding farms, adding gates, unit formations, better pathfinding, etc. It would have also been nice to have some new content (like new maps?). In other words, just a graphics update is not enough.
I hope when the definitive editions of II and III come out, the devs learn this lesson and add some new content to the games besides graphics updates. It is worth the extra money and, in my opinion, would keep people interested longer.
Some suggestions for AoE II: DE
-new and bigger types of maps (how about some mountainous maps, or ones based on Central Asia?; this is critical if 500 pop limit is maintained
-new game modes, especially for multiplayer
-import some of the classic custom scenarios (e.g. Castle Blood Auto)
-minor gameplay fixes, such as:
-set trade route waypoints
-new techs or units